So we see that space (the actual spacetime fabric itself) is enormously stiff (or enormously fluid, but that discussion will come later), but that's not all that's strange about it. It is also expanding. This seems a rather odd coupling of characteristics. Anything that is rigid resists bending or flexing in any way, yet the Universe is clearly expanding at an enormous rate.
This forces us to wonder about the nature of that expansion. When we look to the heavens and see that all of the visible galaxies appear to be moving away from us (regardless of where in the sky we look), it begs the question of how they are moving. This may seem to be yet another odd question. If something is moving away from us, does it really matter how it is moving?
In fact, it does. The question boils down to this: Are galaxies moving through space like cars on an interstate, or are they being carried away like suitcases on an enormous, invisible conveyor belt?
This difference is actually quite important. For example, some large airports provide conveyors that people can step onto to be carried forward rather than walking as normal. These conveyors are usually positioned alongside regular thoroughfares where people can choose to walk as well.
If you take a moment to watch the people moving through such an area, it is clear to see that there is a very simple difference between how the people travel forward. Those on the conveyor belt do not have to move at all - they are simply carried forward; whereas the people walking normally on the floor beside them are passing over the surface of the floor beneath them.
It turns out that this analogy works very well when attempting to understand the nature of how the Universe expands. We still do not know why it is expanding, but we do know a little about how it is expanding. The galaxies are not so much moving through space, they are instead being carried along by space. In fact, Hubble's Law talks about structured Spacetime as being the expanding agent upon which matter is resting. Indeed, it is the very Universe itself that is expanding - the matter within it is not simply moving outward into an empty void of nothingness.
With this, we have finally covered enough points to begin constructing a coherent picture of the question we began with: What is Dark Matter? Over the next few posts, we will begin to put these pieces together. As we do, my hope is that the many enigmas surrounding this hard question of science will begin to come into focus.